Paddy Ashdown, the former Liberal Democrat leader, has made an impassioned plea for party members to avoid "short-term personal manoeuvring" and to stand by Nick Clegg after the first open revolt against his leadership.

Oakeshott, a close ally of Vince Cable, the business secretary, told the Radio 4 Today programme that the Lib Dems need to take a careful look at their "strategy and management" because the party had seen its support halve since the election. Clegg hit back at Oakeshott, telling ITV News: "In politics you always have backseat drivers who sit there making endless comments about people who have to lead."

The moves by Clegg and Ashdown, who anointed the deputy prime minister as his eventual successor more than a decade ago, highlight nerves at the highest levels of the party about the rumbling debate over his leadership.

Oakeshott, a persistent critic of Clegg, advises Cable on an informal but regular basis but his comments prompted warnings by senior Lib Dems that the business secretary is one of two cabinet ministers making plans to succeed Clegg. Ed Davey, the energy and climate secretary, is also said to be "on manoeuvres", according to one well-placed source. Davey dismissed the claim about him as "utter rubbish".

Friends of Cable, who is on holiday and who knew nothing of Oakeshott's interview, denied that he was on manoeuvres. But Cable is aware that the peer has been agitating for some time against Clegg. Cable indicated in an FT interview earlier in the summer that he would be available if there were a vacancy.

Oakeshott decided to speak out after Clegg's interview with the Guardian this week in which the Lib Dem leader called for a wealth tax. The peer welcomed that but was disturbed when Clegg said he would make no attempt to woo back voters on the left who are "lost" to the party.

Cable and Davey will not make direct moves against the deputy prime minister, who told the Guardian that he would lead the party "through the next general election and beyond".

But it is expected they would stand if a vacancy arose in the same way that John Major stood for the Tory leadership in 1990 after Michael Heseltine brought down Margaret Thatcher.

Senior figures are saying that it would take a decisive intervention by a former leader – Sir Menzies Campbell, Charles Kennedy, Lord Steel – or a grandee, such as Lady Williams, for Clegg's position to be undermined. Amid these fears Clegg's original patron has decided to pre-empt any moves against his protege.

In a direct jibe at Oakeshott, Ashdown writes in the Guardian: "Every party dissident, minor or not, suddenly finds themselves welcome on every front page and in every news studio."

The former leader, who was initially wary of Clegg's decision to enter into a coalition with the Tories in 2010, praises Clegg for challenging the party to abandon its "comfort zone" and make the change from being a party of "perpetual opposition" to one of government.

"It is the job of our leader to take us into government," he writes. "I failed; Nick Clegg has succeeded. In my view he has led our party in government, not flawlessly of course, but with a skill no one else in British politics could have matched and a grace under fire which should make us proud. If you want to see how successful he has been, just listen to the complaints from the Tory right."

Ashdown adds that the Lib Dems in government have "mighty things" to achieve over the next year as Britain faces an "existential choice" in confronting the severest economic crisis in half a century. He cites a series of areas, such as "limiting the powers of the state to snoop into our lives" and "protecting our fundamental rights from attack in the name of security" where he says the Lib Dems will make a difference in government. The former party leader writes: "None of this will be achieved by being distracted by midterm summer polls, passing newspaper comments, or short-term personal manoeuvring. The right thing for Liberal Democrats to do now is to continue to do what we have done so well so far. Concentrate on the job we set our hands to under Nick's leadership. Nothing else."

Oakeshott became the first Lib Dem parliamentarian to call publicly for the party to consider a change of leadership, saying: "We have lost over half our market share – if you like to put it that way, if we had been Sainsbury's – since the election. Any business that had done that would be looking very hard now at both its strategy and its management to see how we get some of that back because otherwise we are going to lose a large number of seats at the next election." He made his intervention as part of a discussion on the Today programme with Peter Kellner, of YouGov, which recently found three times more respect for Cable than for Clegg among Lib Dem voters.

Ashdown writes: "Liberal Democrats have long ago learned to ignore the polls and get on with the job in hand. I should know. I am the only political leader in modern British history who has presided over an opinion poll rating represented by an asterisk – denoting that no detectable support could be found for us anywhere in the land!

"We will be judged at the next election by one fact and one fact only. Whether we have had the mettle to stay the course in delivering effective government for our country at a time of crisis. That is the only thing that matters. All the rest is the froth."